Subsonic 300 In Hot Weather

[tslack]

Hey Guys, New to the forum. I loaded some 300 whisper with 220 grain sierra bullets chronographed at 1046fps. I live in Texas with the temp. at 95 degrees and 80% humidity. I am getting a sonic crack with my ar-15 rifle. Is this to fast a round for my conditions. Thanks, Tom

[TheAlmightyBob]

I should probably expect to be corrected for this, so if someone can do better, please do.

The sonic crack is of course the bullet breaking the sound barrier. We all know that. But the sound barrier changes. Both humidity and altitude effect it greatly. If we both fire a round at 1050, but your at sea level and Im in the Blue Ridge Mountains, your bullet will break the sound barrier and mine will most likely not. Not if we do it again, but increase the humidity in the mountains to say 90%, then I will mostly likey break the barrier. So even though your loads are close to the 1050fps mark, you may need to slow it down some since you live in a lower altitude with more humidity. I usually aim for 950 to 1025. But I can get away with close to 1050fps mainly because I live 2700ft above sea level.

[Alleycat]

http://www.sengpielaudio.com/calculator-speedsound.htm
http://www.sengpielaudio.com/calculator-airpressure.htm

[bgrayd]

Originally Posted by tslack

Hey Guys, New to the forum. I loaded some 300 whisper with 220 grain sierra bullets chronographed at 1046fps. I live in Texas with the temp. at 95 degrees and 80% humidity. I am getting a sonic crack with my ar-15 rifle. Is this to fast a round for my conditions. Thanks, Tom

I'm relatively new at this, but from what I've read, you shouldn't be breaking the sound barrier in Texas (where I live also) at 1050 unless it is below freezing. If I am incorrect, please let me know.

[Artful]

If you notice the noise you hear may or may not be sonic boom (ie breaking the speed of sound) as you approach the speed of sound you enter the transsonic area in which the noise of the projectile increases as the bow wave starts to build infront of your bulet.

http://guns.connect.fi/gow/highpow.html

Above is a nice chart relating to bullet noise and velocity that is worth looking at. Transonic is between 1,000 fps and 1,300 fps, and the noise level goes up very, very steeply between those velocities. The noise goes up very slowly between 700 and 1,000 fps, and then takes a dramatic jump to between 90 dB (which is virtually nothing) to almost 140 dB (which is major noise) at 1,300 fps. The noise levels were measured 10 meters to the side of the bullet's flight path. It is nice to see some serious, authoritative studies done on the subject. Measurements were taken all the way up to 3,800 fps, where the noise level increased slightly from that which existed at 1,300 fps.

For measuring this bullet flight sound diagram every .308 cartridge was handloaded prior to each shot. A T8 Scout suppressor is attached to a BR varmint rifle. Sound meter remote readout and loading equipment are shown beside the rifle. Contrary to previous belief it was found, that in practise the speed of sound (Mach 1) was not any sudden threshold to sonic crack. Results of these as well as of other Suppressor Project experiments are published in Alan C. Paulson's book Silencer History and Performance, Vol. 1